1. Field of the Invention
The present invention relates generally to methods and apparatus for calibrating a line card that is configured to be used within a network router. More specifically, the present invention relates to controlling gain adjustment on a line card that includes a gain control circuit.
2. Description of the Related Art
Network routers typically include one or more line cards, and each line card provides an interface between networks that implement different types of data formats. For example, a cable line card is configured to provide an interface between a cable network and a digital network, such as an ATM network. The cable network has data that is encoded within RF modulated signals, while the digital network has data that is encoded within digital signals, such as data packets.
One of the main functions of a conventional line card is to convert one type of data signal into another type of data signal so that two different types of networks may communicate with each other. For example, a cable line card converts modulated RF signals into data packets and vice versa. That is, the data is extracted from a modulated RF signal and formatted into one or more data packets, and data is extracted from data packets and formatted into a modulated RF signal.
However, before the line card can extract data from a modulated RF signal that is input to the line card, the line card is typically configured to amplify the RF signal so that it is strong enough to be processed by the line card's components. That is, the RF signal is amplified such that the line card's internal data processing circuits may properly analyze the RF signal. For example, an analog to digital converter that is configured to convert the RF signal into appropriate digital values requires an RF signal with an amplitude that results in an acceptable range of detectable voltage levels for the RF signal.
Accordingly, conventional RF line cards typically include a gain control circuit for amplifying the RF signal. Each line card is typically calibrated for a given amplitude input prior to installation of each line card into a router. The gain control circuit is used to determine an adjustment value for controlling amplification of a particular input signal to the line card. This adjustment value is then applied to the line card as a fixed adjustment signal for controlling amplification of the particular input signal.
Each line card must be calibrated since each line card's components may perform differently due to small manufacturing variances between separate line cards. That is, the varying characteristics of the components of each line card contribute to how much the input signal is amplified and may result in a different amplification requirement for the input signal for each line card, even though the input signal is identical for each card.
A conventional calibration procedure includes providing an initial adjustment signal to the gain control circuit of the line card. The gain control circuit adjusts the input signal's amplitude based on the initial adjustment signal. This initial adjustment signal is typically set manually while an input signal with a particular amplitude level is input to the line card. The gain control circuit adjusts the amplitude of the input signal according to the initial adjustment signal and outputs an adjusted output signal. The adjusted output signal is analyzed to determine whether it is at an acceptable level. If the adjusted output signal is not at the acceptable level, the initial adjustment signal is altered based on whether the adjusted output signal is too low or too high. If the adjusted output signal is at or near the acceptable level, the current adjustment signal is then fixed and continuously used to adjust any input signal's amplitude that is thereafter received by the line card.
Although conventional calibration techniques for calibrating line cards provide an adequate way to calibrate differing line cards for a known input amplitude, the above described techniques have several disadvantages. Since the adjustment signal is fixed for a known input amplitude during calibration, other input amplitudes may not be used once the line card has been calibrated. That is, once the line card has been calibrated, a user must provide an RF signal having a particular input amplitude to the line card. For example, a 0 dBmV (defined as 1 mV at 75 ohms) RF input signal may be required for proper performance of the calibrated line card.
As a result of these rigid RF input signal requirements, the user may have to amplify or attenuate the RF input signal prior to inputting it to the line card. Thus, the user may have to implement special hardware to meet the input signal requirements. Additionally, each time the system hardware changes (e.g., new cable lines are laid to reach newly built communities), different special hardware is needed for amplifying or attenuating the RF input signal to acceptable levels. In sum, the conventional calibration techniques of providing a fixed adjustment value for a known input amplitude results in the use of significant amounts of additional hardware and corresponding design time.
In view of the foregoing, there is a need for methods and apparatus for calibrating line cards that are flexible and allow a plurality of input amplitudes. Specifically, there is a need for flexible calibration methods and apparatus that do not require significant amounts of hardware redesign and/or development time when the input signal's amplitude changes or the line card is replaced with a different line card.